Method for hardening a nitrided steel



June 16, 1964 P. M. UNTERWEISER METHOD FOR HARDENING A NITRIDED STEELFiled OCt. 3, 1962 7 Sheets-Sheet l 1N VENTOR Pau( M. U/verweLserATTORNEYJ` June 16, 1964 P. M. UNTERwElsER METHOD FOR HARDENING ANITRIDED STEEL 7 Sheets-Sheet 2 Filed OCT.. 3, 1962 INVENTOR /Dul /V[/herwelser J E fw@ l W ATTORNEYS June 16, 1964 P. M. UNTERWEISER METHODFOR HARDENING A NITRIDED STEEL Filed Oct. 3, 1962 BUG/@NJW 7Sheets-Sheet 5 INVENTOR Paul M UmQrwe/ser ATTORNEYS June 16, 1964 P. M.UNTERwElsER 3,137,596

METHOD FOR HARDENING A NITRIDED STEEL Filed Oct. 5, 1962 '7 Sheets-Sheet4 BY/w www m ATTORNEYS June 15, 1964 P. M. UNTERwElsER 3,137,596

METHOD FOR HARDENING A NITRIDED STEEL '7 Sheets-Sheet 5 Filed OCc. 3,1962 QW a. QM.,

ATTORNEYS June 16, 1964 P. M. UNTERWEISER 3,137,596

METHOD FOR HARDENING A NITRIDED STEEL '7 Sheets-Sheet 6 Filed Oct. 3,1962 @www @l IST al QM, mam

INVENTOR Paul M Unerwe/'ser WM Jgfzw 2Q PW NNMMI! ATTORNEYS INVENTOR '7Sheets-Sheet 'T BY//JW J Paul /`7. Umferweser ATTORNEYS June 16, 1964 P.M. UNTERWEISER METHOD FOR HARDENING A NITRIDED STEEL Filed oct. 5, 1962United States Patent C 3,137,596 MEIIiB III-WIDENING A NI'IIIDED STEELPaul M. Unterweiser, Bainbridge Township, Geauga County, Ghia (17680Millbrook Drive, Chagrin Fails, @his Flied er. 3, 1962, Ser. No. 229,269

Claims. (Ci. 148-152) The present invention relates to the hardening ofsteels and more particularly to an improved method for increasing thedegree to which plain carbon, and low-alloy high strength steels can behardened.

When most plain carbon and low-alloy high strength steels are hardenedby the well-known nitriding process, even under optimum conditions, amaximum surface hardness of about Rc 35 for the carbon steels and Rc 58for the alloy steels is about all that can be expected. Even thesehardnessess of Rc 35 and Rc 58 respectively are slightly on the highside and cannot always be obtained consistently on a production basis.When finish grinding or lapping is necessary, the final surface hardnesscan be counted on to be even lower so that in many engineeringapplications this surface hardness limitation severely curtails theusefulness of the nitriding process on the carbon and low-alloy steels.

It has been discovered that the surface hardness of carbon and low-alloysteels, such as for example AISI 1020, 1030, 1041, 1080, 4042, 4130 and4340 can be increased considerably above that heretofore obtainable, byusing conventional nitriding practice followed by rapid heating ofessentially only the nitrided surface layers of the steels to atemperature where austenite is the stable phase, followed by quenching.A satisfactory way to accomplish this rapid heating has been theapplication of a short-time induction heating cycle, for example, forabout two seconds to raise the surface temperature of the steel into theaustenitizing temperature range proper for the alloy composition. Itshould be noted that the effects of the improved process are not limitedto the use of induction heating and that other common methods for rapidheating of the surface such as llame heating, and salt bath heating areapplicable as well.

It is known that when the temperature of a carbon or low alloy nitridedsteel is increased much above 1000 F. the 'nitrogen begins to diluserather rapidly outward from the case and much of the hardnessattributable to the nitriding process itself is lost. By using ashort-time (2-3 second) heating cycle it has proven possible to get thesurface layers of the nitrided steel up to a higher temperature 16001650 F.) with very little loss in nitrogen due to diffusion. A comparisonbetween photomicrographs taken of AISI 4340 nitrided steel before andafter the heating phase indicates quite clearly that after the heatingphase of the nitrided steel was complete, the nitride content of theoriginally nitrided case was considerably increased and that thenitrided case depth has been increased.

The improved method for hardening nitrided steels will become moreapparent from the following description of various examples as appliedto different steel compositions and the accompanying drawings: In thesedrawings:

FIGURE 1 shows the influence of core hardness on the surface hardnessobtained when AISI 4340 steel is nitrided. It is evident that thesurface hardness after nitriding was strongly inliuenced by the hardnessof the steel before the nitriding operation was begun. The higher corehardnesses are reiiected in higher surface hardnesses after nitriding.This is believed to be the effect of more alloying elements beingavailable for nitride formation in the steels tempered at lowertemperatures after quenching to produce higher hardnesses. That is lessalloying element is combined as carbides in this condition ice FIGURE 2shows a comparison of the hardness levels and the range of hardness,reached in both the conventional nitrided condition and after treatmentby the improved process of rapid heating of the nitrided surface layersfollowed by quenching. The improved process produces substantiallyhigher hardnesses to much greater case depths and greatly reduced theeffect of core hardness that has heretofore caused a large variation inthe surface hardness of nitrided low-alloy steels.

FIGURE 3 shows the effect of holding time at the austenitizingtemperature on the hardness measured at various depths within the caseafter quenching of the austenite. The plot shows that the maximumhardnesses (the optimum) are obtained with short holding times thatapproach zero, but that there is-no substantial decrease in the hardnessobtained if holding times shorter than about ten seconds are used.Holding times longer than ten seconds cause unfavorable lower casehardnesses.

FIGURE 4 shows the results obtained with AISI 4130 a chromium containinglow-alloy steel when the improved process of rapidly heating thenitrided surface layers into the austenitizing temperature range,followed by quenching was applied. The plot shows Hardness vs. CaseDepth and curves showing the hardness variation through a conventionalnitrided case on AISI 4130 as well as for the induction surfacedhardened condition are included for comparison. FIGURE 4 shows thesuperior results obtained with the new process :over the separatelyapplied conventional nitriding and surface hardening techniques whenused with AISI 4130.

FIGURES shows the results obtained with AISI 4042 a low alloynon-chromium containing steel, When the improved process was used totreat nitrided specimens. The plot shows Hardness vs. Case Depth afterthe steel was treated by the improved process and for comparison a curveshowing the variation in hardness through the case for AISI 4042 steelin the nitrided condition. FIGURE 5 shows the superiority of theimproved process of rapidly heating the nitrided surface layers into theaustenitizing temperature range followed by quenching when applied tonitrided AISI 4042 steel surface.

FIGURE 6 shows the results obtained with AISI 1041 steel when theimproved process was used to treat previously nitrided specimens. Theplot shows Hardness vs. Case Depth after the steel was treated by theimproved process and for comparison curves showing the variation inhardness through the case for AISI v1041 steel in both the nitrided andsurface hardened conditions. FIGURE 6 shows the superiority of theimproved process of rapidly heating the nitrided surface layers into theaustenitizing temperature range followed by quenching when applied toAISI 1041 over the conventional nitriding and surface hardeningprocesses.

FIGURE 7 shows the results obtained with AISI 1030 a low carbon steel,when the improved process was used to treat previously nitridedspecimens. The plot shows Hardness vs. Case Depth after ,the steel wastreated by the improved process and for comparison curves showing thevariation in hardness through the case for AISI 103() steel in both thenitrided and surface hardened conditions. FIGURE 7 shows the superiorityof the improved process of rapidly heating the nitrided surface layersinto the austenitizing temperature range followed by quenching whenapplied to AISI 1030 over the conventional nitriding and surfacehardening processes.

FIGURE 8 shows the results obtained with AISI 1080, a high carbon steel,when the improved process gas used to treat previously nitridedspecimens. The plot shows Hardness vs. Case Depth after the steel wastreated by the improved process and for comparison curves showing thevariation in hardness through the case for AISI 1080 7 1* steel in boththe nitrided and surface hardened conditions.

FIGURE i8 shows the superiority of the improved process of rapidlyheating the nitrided surface layers into the austenitizing temperaturerange followed by quenching when applied to AISI 1080 over theconventional nitriding and surface hardening processes.

The improved method was first carried out with AISI 4340 because itsnitriding characteristics are Well known and its performance in thenitrided condition 'has been adequately tested, particularly inaircraft.

Test specimens were machined from 1" diameter bar stock. All were copperplated and hardened under protective atmosphere. After hardening,samples were ternpered to produce four core hardness levels: Rc 20-21,24-25, 30-31, and 35-36. All specimens were inspected after temperingand found to be free of any surface decarburization.

The core hardness levels were chosen in order to first determine theeffect of core hardness on nitrided-case hardness. A maximum hardness ofRc 35-36 was not exceeded because the tempering temperature required toproduce this hardness was safely above the nitriding temperature.Consequently, there was no chance of the specimens altering their corehardness during the nitriding cycle.

To insure an optimum nitriding surface, all specimens were cleaned andbonderized prior to nitriding. The specimens were all then nitrided at975 F. for 25 hours in a single-stage cycle. The rate of ammoniadissociation was maintained between 20 and 30 percent throughout thecycle.

The hardness measurements for the nitrided cases at the four corehardness levels are shown in FIGURE 1. The Rockwell c values plottedwere converted from actual N readings. These values are about averagefor nitrided 4340 alloy. As might be expected, lower core hardnessresulted in lower case hardness.

After nitriding, all four specimens were subjected to the same inductionheating cycle. The induction heating unit consisted of a coil of twoturns with an internal diameter measurement of about 1% inches withinwhich the 1" diameter specimens were placed, the coil heating a lengthof about 2" of the specimen. The capacity of the induction unit was 30kw. and its operating frequency was 1,200 kilocycles. The high frequencywas intentionally chosen in order to limit the heat affected zone of thespecimens to approximately the nitrided case area. The time of heatingwas 21/2 seconds which was sufficient to bring the surface temperatureof the specimens up to between 1,600 and 1,650 F. The time at maximumtemperature was not measured but did not exceed a small fraction of asecond. After the induction heating phase was completed, the specimenswere then quenched in oil.

There is no precedent for a tempering `or stress relieving cycle to beused after induction hardening of a nitrided steel. However, it isbelieved that some stress relief would be advisable. Consequently, alltest specimens were held at 400 F. for one hour and air cooled.

Surface hardnesses for all specimens after the induction hardening phasewere in the range of 15N 90-92. The highest surface hardness wasobtained on those specimens with a core hardness of Re 35-36.

Even at the lowest core hardness level (Rc -21) a minimum surfacehardness of 15N 90 was obtained. This is the equivalent of Rc 62 andrepresents an increase over the nitrided surface hardness of almost 20points Rockwell. The maximum spread in surface hardness due to corehardness was eut from 14 to 2 points Rockwell.

The range of hardness Values obtained as a function of Case Depth areplotted in FIGURE 2. The upper limit of case hardness values wereobtained with the Rc 35-36 core hardness. .All case hardness values fellwithin the spread shown, even at the lowest core-hardness level. Thedepth of heat-affected zone due to induction heating was 0.062 inch.

At the upper limit of case hardness, a hardness of Rc l 60 or over ismaintained to a depth of almost 0.020 inch. A minimum of Rc 60 is heldto a depth of 0.012 inch at the lower limit. In both cases, the resultswere found to be unusual and were due to a change in case structure.

The holding time at the austenitizing temperatures may be variedsomewhat without an adverse effect on the results. Experiments with 4340have shown that the holding time can be as long as about ten seconds.This is evident from FIGURE 3 which shows plots of Hardness vs. CaseDepth for different holding times at the austenitizing temperaturebefore quenching of the heated nitrided surface layers was started. Fora minimum holding time such as a fraction of a second or less(approaching zero) one obtains the most favorable Hardness vs. CaseDepth relationship. For holding times longer than about ten seconds theresults are less uniform and substantially lower hardnesses for a givendepth within the case are evident.

In addition to the above described procedures carried out on AISI 4340,similar experiments were conducted on AISI 4130 which is anotherchromium containing lowalloy steel.

AISI 4130 hot rolled bar stock was purchased and given a preliminaryheat treatment by heating to 1550" F. then quenching in water andtempering for four hours at 1020 F. and four hours at F. The bars werethen machined to 1l diameter rounds. A portion of the AISI 4130 steelwas induction surface hardened, and the remainder was nitrided in a twostage process by a treatment of holding at 985 F. for nine hours in anatmosphere of 25-35% dissociated ammonia, increasing the dissociation to65-70% during a time period of about ve hours holding the steel attemperature for an additional forty-six hours under these conditions andfurnace cooling. Some of the nitrided 4130 was examined to measure theHardness vs. Case Depth relationship in the nitrided condition and theremainder of the nitrided portion was subjected to induction heating torapidly heat the nitrided surface layer into the austenitizingtemperature range for this AISI 4130 alloy (l600 F.) and then quenchedwith water.

The induction heating was done with a Lepel High Frequency inductiongenerator, using a power input of 28 kw. and a frequency ofapproximately 300 kilocycles per second. The induction coil was madefrom SAS inch diameter copper tubing flattened to Ma inch and had adouble winding with four turns in each winding. The coil was of 1%@inches inside diameter and one-inch long. A Water spray quenching ringwas located directly below the heating coil and was used to quench thespecimen from the austenitizing temperature.

The surface temperature was measured with a special radiation pyrometerfocused between turns of the coil and connected to a Leeds and NorthrupSpeedomax Type G controller-recorder. The approximate austenitizingtime, i.e., Ael temperature to hardening temperature, was measured fromchart records, using a chart speed of one-inch per second.

After hardening the case depth was measured from depth-hardness data andmacro-etches samples. The case depth hardness measurements were madeusing a Rockwell Superficial Hardness Tester. Rockwell lSN hardnessmeasurements were made on a tapered surface which was very carefullyground on the specimens after heat treatment to expose the hardenedsurface layers. The saine technique was used for the three heat treatedconditions of as nitrided, as induction hardened, and after hardening ofthe nitrided surface.

The hardening temperature used for the AISI 4130 alloy was 1600c F. andthe austenitizing time was approximately 2.3 seconds; the holding timeat the anstenitizing temperature of 1600 F. was essentially zero. Theresults obtained with 4130 steelare shown in FIGURE 4 where Curves (a),(b) and (c) show the results of the Hardness vs. Case Depth measurementsfor the nitrided,

^ e3 induction hardened, and nitrided plus heating `and quenchingconditions. Curve (c) in FIGURE 4 shows the superior results obtainedwith the improved process when compared with the conventional results.

The invention has also been found to be applicable to other low-alloysteels which do not contain chromium as an alloying element such as, forexample AISI 4042. In this case preparation of the sample, nitriding,and hardening were carried out in thesame manner as for 4130 and theresults are shown in FIGURE 5 which illustrates the favorable resultsobtained when the improved process is applied to AISI 4042. The curvesin FIGURE 5 show the Hardness vs. Case Depth relationship for 4042 steelhardened by (a) nitriding and (b) the improved process of rapidlyheating the nitrided surface layers into the austenitizing temperaturerange followed by quenching. The superior hardness results obtained bythe use of the improved process are shown in Curve (b).

In addition to the low-alloy steels, experiments have confirmed the factthat similar favorable results are obtainable when the improved process,namely rapid heating of a nitrided surface into the austenitizingtemperature range followed by quenching is applied to low, medium andhigh plain carbon steels that have been nitrided.

As an example of a medium carbon steel, a sample of AISI 1041 wasprocessed as follows: I-Iot rolled bar stock was purchased and given apreliminary' heat treatment by heating to 1570 F., then quenching inwarm oil and tempering at ll00 F. for four hours. In this case thenitriding was carried out as described for the AISI 4130 steel above.The heating and quenching equipment used was the same as for the 4130.rIhe approximate austeriitizing time (heating above the Ael) was threeseconds; the maximum austenitizing temperature was about 1625 F. I hesamples were water quenched after the induction heating cycle. Thefavorable results obtained with the improved process when applied tonitrided AISI 1041 are shown in FIGURE 6. The curves in FIGURE 6 showthe Hardness vs. Case Depth relationship for AISI 1041, steel hardenedby nitriding (Curve a), induction hardening (Curve b) and the improvedprocess of rapidly heating the nitrided surface layers into theaustenitizing temperature range followed by quenching. The superiorhardness results Obtained by the use of the improved process are shownin Curve (c) of FIGURE 6.

As an example of a low carbon steel, a sample of AISI 1030 was processedas follows: one inch diameter hot rolled bars were purchased and given apreliminary heat treatment by heating to 1600 F.,.quenching in water andtempering at ll00 F. for four hours. The hardness after this treatmentwas 64 RlSN. Some of the heat treated 1030 steel was machined to adiameter of 7s inch and nitrided by heating to 985 F. and holding for 15hours 20% dissociated mmonia, increasing the dissociation to 80% in aneight hour period, holding for an additional forty-two hours and thenfurnace cooling. The surface hardness after nitriding was 71 RlSN. rIhevariation in hardness through the nitrided surface layers was measuredand the results are shown in Curve (a) of FIG- URE 7.

Another sample of AISI 1030 which had bee the preliminary heat treatmentdescribed above and machined to 7A, inch diameter but not nitrided, wasinduction hardened. The induction hardening was done using a Lepel HighFrequency induction generator at a power input of kw. and a frequency ofapproximately 300 kilocycles per second. The induction coil andquenching method used were the same as for the AISI 4130 steel. Thesurface temperature was measured with a Chromel-Alumel thermocouplepercussion welded to the specimen surface. The temperatures and timeswere recorded on a Leeds and Northrup Speedomax G controller-recorder asinthe AISI 4130 experiments. A hardening temperature of l700 F. was usedand the heating time was 2.5 seconds. The hardness variation through thecase was measured and the ti results are shown in Curve (b) FIGURE 7.The maximum surface hardness measured Was 87 RlSN.

A portion of the nitrided AISI 1030 was treated by the improved processusing the same temperature and time as for induction hardening. Inaddition to the water quenching the sample was immersed in liquidnitrogen for twenty minutes. The results of hardness measurementsthrough the case produced by the improved process are shown in Curve(c), FIGURE 7. The superior results obtained are evident in FIGURE 7 andthe maximum surface hardness measured was 92 RlSN. The increased casehardness over the induction hardened test specimen is maintained to atleast a depth of 0.050 inch after this treatment was 70 RlSN. Thenitriding processV was the same as used for AISI 1030. The austenitizingtemperature used for induction hardening of the nonnitrided and nitridedspecimens was 165 0 F. and the heating time was 2.5 seconds. The sampleswere then quenched in the same manner as the AISI 1030 specimen.

The results obtained with AISI 1080 when processed by the improvedmethod are shown in FIGURE 8. Curve (a) shows the Hardness vs. CaseDepth for 1080 steel in the nitrided condition, (b) the Hardness vs.Case Depth for induction hardened 1080 steel and (c) the superiorHardness vs. Case Depth for 1080 steel processed by the improved processof rapid heating of a nitrided surface into the austenitizingtemperature range following quenching. The maximum surface hardness of77.5 RlSN in the nitrided condition has been raised (by treatment withthe improved process) to 92.5 R15N which is superior to the 91 R15Nobtained by induction hardening alone. The increased hardness of thecase produced with the improved process over the induction hardened casedepth beyond is maintained to the depth of the original nitrided case.

Similar favorable results as regards increased hardness and depth ofcase were obtained in experiments carried out on nitrided samples ofAISI 1020, AISI 1030, and AISI 1080, that were treated by theimprovedprocess but not immersed in liquid nitrogen after quenching to roomtemperature.

The extra hardening effect produced by the rapid heating into theaustenitizing temperature rangel and quenching of the nitrided surfacelayers is believed to be a combined el'lect of two hardening mechanisms:

(a) In the conventional nitrided case, the ability of Vthe alloyingelements to form carbides is known to influence case hardness. As thetempering temperature is increased, more and more of a carbide-formingelement is precipitated as a carbide, leaving less in solution in theiron for nitride formations. Nitralloy steels containing aluminum arenot somarkedly aected in this way since virtually all of the elementremains in solution and is available for nitride formation.

Depending upon the amount of precipitated carbide, for example, intempered 4340, nitride case hardness can vary within a fairly broadrange when the steel is nitrided in the conventional manner. Itapparently cannot exceed a maximum hardness of about Rc 58 because toomuch of the carbide-forming elements have already been precipitatedprior to m'triding. The short heating cycle raises the surfacetemperature of the steel to 1600"- 1650 F. momentarily. This treatmentis sucient to place carbides back into solution thus releasingcarbideforming elements for further chemical combination.

(b) A second eifect of heating the nitrided surface layers into theaustenitizing temperature range is to effect the solution of some of thenitrides (as well as carbides) so that nitrogen would be dissolved inthe austenite as an alloying element. On quenching of the nitrogenenriched austenite the dissolved nitrogen would remain in solution andon transformation of the austenite produce nitrogen rich products ofdecomposition of increased hardness. The dissolved nitrogen would behavein a manner similar to carbon as an interstitial element to promote ahardening effect. This is particularly evident in the carbon steelswhere little nitride forming elements are present, so that anincreasedhardness because of solution of carbide forming elements otherthan iron to cause an increased dispersion of nitrides would not beexpected, and further the increased hardness effect caused by thenitrogen on heating and quenching has been shown to be dependent on thehardenability (i.e., the carbon content) of the steels. The maximum gainin hardness achieved over the induction hardened samples was seen in the1020 steel, and the incremental hardness decreased, although the maximumhardness reached increased, as the carbon content was increased from0.20 through 0.30 to 0.80%.

The properties of the quenched structure produced by heating andquenching of a nitrided surface further suggests that the nitrogen hasentered into the hardening reaction as other than an increaseddispersion of nitrides. This was shown by an increased resistance tosoftening, i.e., tempering, of the quench hardened nitrided case onreheating. This shows that the product of this hardening reaction is ofa different nature to the normal carbon martensites. The difference isbelieved to be because of the dissolved nitrogen which enhances thehardenability of the austenite.

Both of the above mechanisms (a) and (b) cause the increased hardnessmeasured in the specimens treated by this new hardening process; theproportional increase caused by each is dependent on the alloy contentand primary hardenability of the steel treated. The effect of increaseddispersion of the alloy nitrides was seen to be minimized in the carbonsteels, Where the alloying effect of the nitrogen on the hardness of thetransformed austenite was the greatest.

This application is a continuation-in-part of my copending applicationSerial No. 819,627, led lune 11, 1959.

l claim:

1. The method of treating a steel which has been nitrided which.comprises the steps of rapidly heating substantially only the nitridedportion of the steel to a temperature within the austenitizing range forthe steel, holding the steel at the austenitizing temperature for arelatively short time ranging from about ten seconds down to andapproaching zero, and then initiating and continuing quenching until atemperature is reached such that substantially all of the austenite inthe nitrided portion is transformed into nitrogen bearing martensite.

2. The method as defined in claim 1 of treating a steel which has beennitrided wherein the total time for heating the steel and for holding itat the austenitizing temperature does not exceed about thirteen seconds.

3. The method as defined in claim l of treating a steel which has beennitrided wherein the heating time is not in excess of about threeseconds.

4. The method of treating a steel which has been nitrided, said nitridedsteel being selected from the group consisting of plain carbon steelsand low alloy steels which comprises the steps of rapidly heatingsubstantially only the nitrided portion of the steel to a temperaturewithin the austenitizing range for the steel, holding the steel at theaustenitizing temperature for a relatively short time ranging from aboutten seconds down to and approaching v zero, and then initiating andcontinuing quenching until a temperature is reached such thatsubstantially all of the austenite in the nitrided portion istransformed into nitro-,

gen bearing martensite.

5. The method of treating a steel which has been nitrided, said nitridedsteel being a chromium containing low alloy steel, which .comprises thesteps of rapidly heating substantially only the nitrided portion of thesteel to a temperature within the austenitizing range for the steel,holding the steel at the austenitizing temperature for a relativelyshort time ranging from about ten seconds down to and approaching zero,and ten initiating and continuing quenching until a temperature isreached such that substantially all of the austenite in the nitridedportion is transformed into nitrogen bearing martensite.

6. The method of treating a steel which has been nitrided, said nitridedsteel being a non-chromium containing low alloy steel, which comprisesthe steps of rapidly heating substantially only the nitrided portion ofthe steel to a temperature within the austenitizing range for the steel,holding the steel at the austenitizing temperature for a relativelyshort time ranging from about ten seconds down to and approaching zero,and then initiating and continuing quenching until a temperature isreached such that substantially all of the austenite in the nitridedportion is transformed into nitrogen bearing martensite.

7. The method of treating a steel which has been nitrided, said nitridedsteel being selected from the group consisting of low carbon steelsincluding AISI 1020 and AISI 1030 which comprises the steps of rapidlyheating substantially only the nitrided portion of the steel to atemperature within the austenitizing range for the steel, holding thesteel at the austenitzing temperature for a relatively short timeranging from about ten seconds down to and approaching zero, and theninitiating and continuing quenching until a temperature is reached suchthat substantially all of the austenite in the nitrided portion istransformed into nitrogen bearing martensite.

8. The method of treating a steel which has been nitrided, said nitridedsteel being a medium carbon steel including AlSI 1041, which comprisesthe steps of rapidly heating substantially only the nitrided portion ofthe steel to a temperature Within the austenitizing range for the steel,holding the steel at the austenitizing temperature for a relaitvelyshort time ranging from about ten seconds down to and approaching zero,and then initiating and continuing quenching until a temperature isreached such that substantially all of the austenite in the nitridedportion is transformed into nitrogen bearing martensite.

9. The method of treating a steel which has been nitrided, said nitridedsteel being a high carbon steel including AiSl 1080, which comprises thesteps of rapidly heating substantially only the nitrided portion of thesteel to a temperature within the austenitizing range for the steel,holding the steel at the austenitizing temperature for a relativelyshort time ranging from about ten seconds down to and approaching zero,and then initiating and continuing quenching until a temperature isreached such that substantially all of the austenite in the nitridedportion is transformed into nitrogen bearing martensite.

10. The method of treating a chromium-containing, low alloy steel whichhas been nitrided which comprises the steps of raising the temperatureof substantially only the nitrided portion of the steel to a temperatureWithin the austenitizing range of from about 1600 to 1650 F. Within ashort time not exceeding 2-3 seconds, initiating quenching within afraction of a second after attaining said austenitizing temperature, andcontinuing the quenching to a temperature below that at which martensiteis formed so as to convert substantially all the austenite intomartensite containing nitrogen as an alloying constituent.

References Cited in the file of this patent UNITED STATES PATENTS

1. THE METHOD OF TREATING A STEEL WHICH HAS BEEN NITRIDED WHICHCOMPRISES THE STEPS OF RAPIDLY HEATING SUBSTANTIALLY ONLY THE NITRIDEDPORTION OF THE STEEL TO A TEMPERATURE WITHIN THE AUSTENITIZING RANGE FORTHE STEEL, HOLDING THE STEEL AT THE AUSTENITIZING TEMPERATURE FOR ARELATIVELY SHORT TIME RANGING FROM ABOUT TEN SECONDS DOWN TO ANDAPPROACHING ZERO, AND THEN INITIATING AND CONTINUING QUENCHING UNTIL ATEMPERATURE IS REACHED SUCH